The invention relates to a color display tube comprising an evacuated envelope consisting of a substantially rectangular display window connected to a conical portion coupled to a neck, a display screen on the inside of the display window, which display screen comprises triplets of stripe-shaped regions which can luminesce in three colors and extend substantially perpendicularly to the long sides of the display window, and shadow mask provided in front of and parallel to the display screen. The mask comprises a metal sheet having rows of slot-shaped apertures extending substantially parallel to the stripe-shaped regions and separated from each other by ridges and elongate in the direction of the rows; one row of apertures being alotted to each triplet. The invention relates more particularly to such a display tube comprising, in addition, in the neck, and electron gun system including three electron guns which are situated with their axes in a plane extending substantially perpendicularly to the stripe-shaped regions and which serve to generate three electron beams which converge on the display screen and are deflected over the display screen and each impinges upon stripe-shaped regions of one color via the apertures in the shadow mask; said shadow mask having a non-rotationally symmetrical transmission variation proceeding from the center towards the edge.
Such a color display tube is disclosed in German Patent Application No. 27 17 441 laid open to public inspection. The shadow mask described in said patent application comprises apertures which become smaller from the center towards the edge of the shadow mask. Apertures having equal dimensions are situated on non-circular curves which are substantially rectangular with rounded corners and which follow substantially the edge of the shadow mask. The advantages of such a variation of the dimensions of the apertures in the shadow mask and, hence, of the mask transmission according to this patent application are a more uniform brightness variation which is adapted to the shape of the display screen and, in addition, an improvement of the color purity.
U.S. Pat. No. 3,686,525 discloses a color display tube having a shadow mask with rows of circular apertures situated on barrel-shaped lines in the horizontal direction and on pin-cushion-like lines in the vertical direction. According to this Patent, the dimensions of the apertures must have such a variation that a circular variation of the transmission from the center of the mask is obtained. In other words, the "equitransmission factor curves" are circles.
Nowadays, it is desired to provide display tubes with which a picture having a larger brightness can be displayed. Such display tubes as a matter of fact are used in an ambience with much ambient light. Moreover, such tubes provide a sharper picture at smaller values of the ambient light. This gain in definition is obtained because a smaller cathode current than usual is necessary to obtain the same brightness of the picture so that the diameter of the electron beam is reduced. Such display tubes can be obtained by making the mask transmission larger. The mask transmission T.sub.M is defined as the ratio between the area of an aperture and the mask surface per aperture so that it holds approximately that: ##EQU1## wherein: b is the width of the elongate aperture
l is the length of the elongate aperture PA1 X is the distance between two rows of apertures, and PA1 Y is the distance between two apertures in a row.
This enlarging of the mask transmission is carried out by making all apertures in the shadow mask wider. However, this has a negative influence on the color purity or the accuracy with which the spots of the electron beams, which spots are reproductions of the apertures, coincide with the stripe-shaped regions. The value of the displacement which a spot can undergo on a stripe-shaped region without influencing the color purity is termed guard band. Guard band is necessary to compensate for displacements of the spot which are caused, for example, by differences per tube within the production tolerances or by mask displacements as a result of thermal effects.